Coal contains relatively low levels of mercury ranging from 0.010 to 3.5 parts per million by weight. Mercury emissions from coal combustion in power plants however have received special attention since coal is burned in about 1700 power plants in the United States (U.S.). In 1999, 43 tons of mercury were emitted from U.S. coal fired power plants. Mercury is classified as a hazardous air pollutant under the Clean Air Act Amendments (CAAA) of Title III and is an air toxic. Air toxics are hazardous air pollutants which are known or suspected to cause cancer or other serious health effects such as birth defects or reproductive effects. In the 1998 Utility Air Toxics Report to the U.S. Congress the Environmental Protection Agency (EPA) indicated that mercury posed the greatest concern of all the toxics emitted from power plants.
In 2003, the EPA announced its intention to regulate power plant mercury emissions. Final regulations are expected in 2005, and compliance is projected for 2007. The new regulations could require up to a 90% reduction in mercury emission from power plants. This requirement will have significant financial implications for power plants. Therefore, developing cost effective control technologies to remove mercury from power plant emissions will become increasingly necessary.
Several control technologies are currently being studied. These technologies focus mainly on pre-combustion and post combustion processes. Pre-combustion processes involve coal-cleaning technologies to reduce mercury burden at the source. Post combustion processes involve adsorption of mercury vapor by carbon-base material and conversion of that vapor to an oxidized form for removal by a wet system or particulate filters. These technologies convert the mercury contaminant from a gas to a solid. Therefore, it is still necessary to dispose of the solid phase as hazardous waste.
It has been estimated that 90% of mercury emitted from burning western coal is in the form of mercury vapor (elemental). Eastern coal combustion generates 60% elemental mercury and another 40% mainly in the form of divalent mercury. Regulation of these mercury emissions is near and thus the development of effective technologies to control these emissions is critical.
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